In response to the need for safety during impacts and the need for reductions in the weight of automobiles, the strength not only of structural members of automobile bodies but also of steel sheets suitable for outer panels of automobiles such as side panels, hoods, doors, and fenders is being continuously increased. These steel sheets require not only a good surface appearance but also excellent press formability and particularly excellent drawability. From the past, it has been known that drawability has an extremely high correlation with the Rankford value (r value), which is a plastic strain ratio specified by JIS Z 2254, and that the higher the r value, the better is drawability. Therefore, this value has been widely employed as an index of drawability and has been widely used as an index in material design.
According to the investigations of the present inventors, blanks which are used for outer panels of automobiles such as side panels, hoods, doors, and fenders are among the largest blanks in size used for automotive parts, and nearly rectangular blanks which have been cut from a wide coil to keep the same width are subjected to pressing. In the case of a side panel, for example, the four corners of an opening which are difficult to press-form are positioned at 45° with respect to the rolling direction. If the r value in a direction at 45° with respect to the rolling direction (hereinafter referred to as r45) of a steel sheet as a material being worked is low, wrinkles and cracks may easily develop. Accordingly, it is important for the steel sheet to have an increased value of r45.
An effective method of producing a steel sheet having a high r value is to add a carbonitride-forming element such as Ti or Nb to an ultralow carbon steel having a C content of at most 30 ppm. Such a steel sheet called IF (interstitial free) steel is widely used generally as mild steel. Steel sheets having a high r value and high strength have been developed based on IF steel by adding solid solution strengthening elements such as Mn and P. However, solid solution strengthening elements are generally expensive and lead to cost increases of steel sheets. Therefore, Patent Document 1 discloses a technique using precipitation strengthening by NbC and TiC with the object of reducing solid solution strengthening elements. Patent Document 2 discloses a steel sheet having improved surface properties and mechanical properties by adding a suitable amount of Nb to a steel sheet containing 0.0040-0.01% of C so as to form fine precipitates of NbC and thereby refine the structure.
However, addition of Nb and Ti causes dense formation of fine precipitates of NbC and TiC or complexes thereof, i.e., (Nb,Ti)(C,N). Therefore, depending upon hot rolling conditions, grain growth sometimes becomes poor due to the pinning effect which obstructs the movement of grain boundaries at the time of recrystallization, thereby possibly causing a decrease in the r value.
For a high tensile strength cold-rolled steel sheet to which primarily Ti is added, it is difficult to have an increased r45 value. When such a steel sheet is used for outer panels of automobiles, press cracks and wrinkles often develop, leading to an increase in pressing defects. In a high tensile strength cold-rolled steel sheet to which primarily Nb is added as described in Patent Document 2, since NbC precipitates more finely than TiC, high temperature annealing becomes essential. In order to avoid high temperature annealing, it is necessary to coarsen the precipitates, but the usual technique therefor, i.e., high temperature coiling at the time of hot rolling is inadequate.
Namely, in order to increase r45 of a steel sheet, it is necessary to optimize the form of precipitates which are present therein and the ratio of the Nb and Ti contents.
Patent Document 3 discloses a cold-rolled steel sheet based on a mild steel having a low strength in which in order to improve the planar anisotropy of the r value, the steel has a decreased Al content and Mg and Ti are added such that the size and surface density of extremely fine oxides of Mg and Ti having a size of at most 0.1 μm contained in steel are controlled so as to densely disperse such oxides. This technique, however, refines oxides by the action of Mg, and it does not control precipitation of carbonitride-forming elements such as Nb and Ti which have a large effect on the r value. Furthermore, addition of Mg, which has extremely high reactivity, to molten steel at the time of melting so as to uniformly disperse its oxide is extremely difficult and causes problems from an operational standpoint.
Patent Document 4 discloses a steel sheet of an ultralow carbon steel having a reduced Al content and containing Ti and a process for its manufacture. This manufacturing process controls the composition of inclusions such that they are fine, they do not contain a locally crystallized hard phase, and they as a whole are readily deformed and crushed. As a result, the incidence of inclusion defects decreases, and the sol. Al content of the steel is decreased, so a steel having a low recrystallization temperature and high press formability can be obtained. However, there is no mention of the effect of inclusion defects on the surface appearance or formability, and in particular there is no mention of how to improve the r value, which is the most important index of drawability.
Patent Document 1: JP 10-46289 A1
Patent Document 2: JP 2000-303145 A1
Patent Document 3: JP 11-323476 A 1
Patent Document 4: JP 10-226843 A 1